1. Field of the Invention
The present invention relates to an image forming method utilizing thermal development. More particularly, the invention relates to an image forming method with satisfactory density and stable color tone of an image even at high-speed continuous processing.
2. Description of the Related Art
In recent years, there has been a strong desire in the medical field to reduce the amount of used processing liquids in consideration of environmental safety and space saving. For this reason, there is a need for a technology in the field of photothermographic material for medical diagnosis and for photographic applications, capable of efficient exposure with a laser image setter or a laser imager and of forming a sharp black image with high resolution and sharpness. Such photothermographic material can eliminate use of processing chemicals in solutions and can provide users with a thermal development system which is simpler and does not pollute the environment.
Although there is a similar need in the field of ordinary image forming materials, an image for medical use requires particularly high image quality excellent in sharpness and granularity since minute image presentation is necessary. Further, an image of cold black tone is preferred in consideration of ease of diagnosis. Currently various hard copy systems utilizing pigments or dyes, such as an ink jet printing system and an electrophotographic system, are available as ordinary image forming systems, but such systems are not satisfactory to be used as medical-use image output systems.
On the other hand, a thermal image forming system utilizing an organic silver salt is disclosed (for example, U.S. Pat. Nos. 3,152,904 and 3,457,075, and “Thermally Processed Silver Systems”, D. Klosterboer, Imaging Processes and Materials, Neblette 8th edition, edited by Sturge, V. Walworth and A. Shepp, Chap. 9, p.279(1989)). More specifically, a photothermographic material (hereinafter also called a “photosensitive material”) has a photosensitive layer in which a photocatalyst (for example, silver halide) of a catalytic active amount, a reducing agent, a reducible silver salt (for example, organic silver salt) and a toning agent for regulating silver color tone if necessary, are generally dispersed in a matrix binder. After image exposure, the photothermographic material is heated to a high temperature (for example, 80° C. or higher) whereby a black silver image is formed by a redox reaction between the silver halide or reducible silver salt (acting as an oxidizing agent) and the reducing agent. The redox reaction is accelerated by a catalytic effect of a latent image on silver halide, formed by light exposure. Therefore, a black silver image is formed in an exposed area, which is disclosed in various references (refer to, for example, U.S. Pat. No. 2,910,377 and JP-B No. 43-4924). As a medical image forming system based on a photothermographic material utilizing such principles, Fuji Medical Dry Imager FM-DPL has been launched on the market (refer to, for example, Fuji Medical Review No. 8, p.39–p.55 (1999)).
A photothermographic material including a photosensitive silver halide and a non-photosensitive organic silver salt has high sensitivity and is a material suitable as an image recording material for the aforementioned laser output, and applications in such a field are anticipated to further increase hereafter. For expansion of such applications and increase in the process amount, further improvements are required in image recording speed and developing speed, improved adaptability to installation of the apparatus and the environment, and reduction in size of the entire apparatus including an optical system for laser exposure and a thermal development unit.
There has been developed and widely utilized an advanced image recording apparatus integrating a laser exposure unit and a thermal development unit and not requiring water supplying and discharging pipes and an exhaust pipe for discharged gas (refer to, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075, and “Thermally Processed Silver Systems”, D. H. Klosterboer, Imaging Processes and Materials, Neblette 8th edition, edited by J. Sturge, V. Walworth and A. Shepp, Chap. 9, p.279(1989)). In such an apparatus, a photothermographic material is at first transported to a laser exposure unit, and after an image data is recorded by scanning exposure, the photothermographic material is guided to a heat development unit for heating to form an image. Thereafter it is cooled and discharged from the apparatus. However, since the laser exposure unit and the thermal development unit are sufficiently separated from each other in order to avoid mutual detrimental influence, the entire apparatus lacks compactness in size and requires a large space for installation.
One way of reducing the size of the total system is to place the laser exposure unit and the thermal development unit closely. Conventionally, the heat development unit is equipped with a heat source for uniform heating at around 120° C., and is formed with a material of high heat capacity at high temperature in order to reduce temperature variation. Furthermore, in order to prevent heat leakage, it is entirely covered with a heat insulating material. However, in order to prevent the temperature in the laser exposure unit from rising caused by heat conduction by a recording material or by heat diffusion by leaking air, the thermal development unit and the laser exposure unit are designed with sufficient mutual distance. The principal object of preventing the temperature increase of the laser exposure unit is to maintain the precision of the optical system. Particularly in the case of photothermographic material, in addition to thermal aberration in the precision of the optical system, another factor is a stain of the optical system by volatile substances generated by heating.
Designing of a photothermographic material with reduced generation of volatile substances under heating, for the purpose of reducing the size of the apparatus, has solved the drawback of stain in the optical system, but another drawback has been newly discovered. In the case of continuously executing exposure and thermal development of the photothermographic material in an apparatus in which a laser exposure unit and a thermal development unit are closely positioned with each other, sensitivity of a first photosensitive material and that of a last (for example, the 20th) photosensitive material are clearly different. It is beleaved that such a change is generated gradually and continuously, but an apparent difference from the first material is clearly recognized at approximately the 20th material.
Since a system is required to exhibit a constant sensitivity, this is an important issue in reducing the size of the apparatus. Even if such problems are caused by a local temperature variation in the apparatus, a photothermographic material that is not affected by such variation is required.
Concomitant with the spread of CT, MRI and CR, many medical images are outputted, requiring processing of many medical images within a short period of time. Under such a situation, there is a strong desire for higher processing ability of the medical dry imager. On the other hand, in the dry imagers utilizing a thermal development method by a plate heater or a drum heater, higher stability of the thermal development unit is desired for achieving higher processing ability. As the temperature at the thermal development unit is lowered by thermal development of one sheet of photosensitive material, such temperature lowering must be recovered before the next sheet of photosensitive material arrives. A local temperature variation may be reduced by increasing the heat capacity or volume of the thermal development unit, particularly a heater drum, but such a method leads to a larger size of the apparatus, longer start-up time of the apparatus, and increased electric power consumption, thus undesirable in terms of use and cost. Therefore, there has been a need for an image forming method which uses a photothermographic material and is not susceptible to temperature variation, and is capable of forming a stable image without being easily affected by the instability in temperature at the thermal development unit in such high-speed processing.